The superheat-controlled expansion valve needs no defense in its use in comfort air conditioning applications and in commercial refrigeration, but it is appropriate now to examine its applicability to industrial refrigeration systems. Many respected industrial refrigeration professionals have in the past made flat assertions that the superheat-controlled valve should not be used in low temperature, ammonia refrigeration systems. The next few paragraphs will
present some reasons8 why those two conditions present challenges for this type of expansion valve. In addition, certain applications will be proposed where this type of expansion valve could be properly applied. One drawback of superheat-controlled valves for low-temperature applications is the high superheat that occurs at low temperatures with the consequent reduction in the effectiveness in the use of the evaporator surface.
This characteristic is particularly applicable to systems where the evaporating temperature is frequently pulled down from a warm state. Fig. 11.18a, for example, shows a valve serving an R-22 system with R-22 as the power fluid. The valve is set at 5°C (9°F) superheat at a high temperature of 5°C (41°F) which provides a 100-kPa (14.6-psi) pressure difference across the diaphragm.
To open the valve the same amount at a low temperature of -30°C (-22°F), a superheat of 12°C (22°F) is required. This large amount of superheat will penalize the performance of the evaporator. The situation can be avoided by selecting a type of valve that uses what is called a cross charge for the power fluid, as illustrated in Fig. 11.18b. This cross charge is a specially blended mixture whose pressure-temperature curve is precisely displaced from the curve of the system refrigerant. In this way a constant relationship of delta p to superheat over a wide range of temperatures prevails.
One of the critical drawbacks of the superheat-controlled expansion valve for low-temperature systems is that superheat in the evaporator becomes a progressively greater deterrent to the maintenance of capacity and coefficient of performance as the evaporating temperature drops. In fact, the improved evaporator heat-transfer characteristic, as shown in Fig. 8.6 and 8.7, was one of the reasons for applying liquid recirculation. The penalty on the power and capacity of each degree of temperature becomes more prominent in lowtemperature systems.
The superheat-controlled valve encounters another difficulty when operating at low temperatures, and that is that there may be no temperature available that is high enough to provide the superheat needed to open the valve. As Fig.
11.15 shows, a superheat of perhaps 7°C (12.6°F) may be required to fully open the valve. An air coil in a frozen-food storage facility may be designed for a 5.5°C (9.9°F) temperature difference between the inlet air and the refrigerant. The entire coil is surrounded by air at the entering temperature, so there is no source of air at a high enough temperature to provide the required superheat. The drawbacks of the superheat-controlled valve in industrial refrigeration systems that operate at low temperature are formidable, but at medium temperature levels—in the neighborhood of 0°C (32°F)—the temperature level alone is not a major issue.
Consider next the question of the compatability of ammonia with direct expansion systems in general and with superheat-controlled expansion valves in particular. Refrigeration practitioners occasionally report attempts to install a superheat-controlled valve on a coil designed for liquid recirculation. Invariably the coil is not able to provide as high a heat transfer rate as with liquid recirculation, and the reason is that the liquid ammonia flows along the bottom of the tubes without wetting the entire tube wall. Section 6.27 when discussing direct-expansion ammonia air-cooling coils stressed the need of small-diameter tubes with long circuit lengths in order to achieve sufficient agitation of the boiling refrigerant to adequately wet the surfaces of the tubes. Interest is high in successful application of direct expansion to ammonia coils, because of the lower refrigerant charge and the reduction in first cost by eliminating liquid pumping equipment.
Ammonia possesses five or six times the latent heat of halocarbon refrigerants, and this property is almost always a valuable asset. With control valves, however, the high latent heat may be a drawback, because the flow rate of ammonia for a given refrigerating capacity will be low. Ammonia expansion valves are built with much smaller ports than those for halocarbons refrigerants, and at their nearly closed position a minute change in stem position results in large percentage changes in flow rate. Expansion valves in ammonia systems are thus more prone to instability than in halocarbon systems. Another feature of ammonia superheat-controlled expansion valves is that the outlet line of the evaporator to which the bulb is strapped is likely to be steel. The thermal conductivity of steel is about one-eighth that of copper, so the response of the ammonia valve is likely to be more sluggish.
An approach to circumventing the control instability that occurs with a nearly closed control valve is to resort to pulse-width modulation of a valve that is either completely open or completely closed. A signal, such as the superheat of the refrigerant leaving the evaporator, is fed to a microprocessor which regulates the pulse width. Figure 11.19 shows three different percentages of flow rates that are accomplished by varying the fraction of time that the valve is open during the uniform pulse width. A typical cycle time is 6 s. This valve is essentially a rapidly opening and closing solenoid valve which will survive for millions of cycles.